Method of forming a hard surface on cemented carbides and resulting article

ABSTRACT

CEMENTED CARBIDE CUTTERS ARE TREATED WITH BORON CARBIDE AND HYDRIDE OF A REACTIVE OR REFRACTORY METAL AT TEMPERATURES ABOVE 500*C. THIS PROCESS HARDENS THE SURFACE OF THE CUTTER AND DECREASES WEAR IN MACHINING OF VARIOUS METALS.

METHUD F FORMING A HARD SURFACE 0N CEMENTED CARBTDES AND RESULT- ENG ARTHCLE Herbert S. Kalish, Short Hills, N1, assignor to Adamas Carbide Corporation, Kenilworth, NJ. No Drawing. Filed Apr. 14, 1971, Ser. No. 134,042 int. Cl. B22f 3/24, 7/02 US. Cl. 29-1822 14 Claims ABSTRACT 9F THE DESCLQSURE Cemented carbide cutters are treated with boron carbide and hydride of a reactive or refractory metal at temperatures above 500 C. This process hardens the surface of the cutter and decreases Wear in machining of various metals.

This invention relates to a surface treatment on that class of materials called cemented carbides or hard metals, including cemented tungsten carbide and cemented titanium carbide, with or Without the addition of other carbides and binders. The surface treatment involves the diffusion of certain elements from specific compounds surrounding the material as specified in the examples in this invention, said diffusion of elements into the cemented carbide resulting in the formation of compounds within the structure of the surface of the cemented carbide, which hardens the surface and changes the characteristics of the surface such that cemented carbide tools, made with this surface treatment, show decreased wear in the machining of various metals.

The previous methods of improving the surface of cemented carbides have involved a process for putting a coating of titanium carbide or titanium nitride on a surface of cemented carbide, by utilizing a gas phase process which results in an overlay of the hard material on the cemented carbide insert. Most of these methods involve complex equipment, relatively high temperatures, and frequently suffer from the difficulty of working with highly corrosive titanium tetrachloride gas as a starting material, and hydrogen chloride gas as a byproduct of the reaction. My invention involves the use of readily available materials which do not have corrosive or other dangerous characteristics.

'In addition, the surface treatment that is obtained by this invention, is superior to those coatings, or treatments, which have been previously described through patents or in the literature before. Its superiority stems from the fact that the treatment involves a formation on the surface and diffused a minute distance into the surface of a boride of totaniurn TiB or a carbide of totanium (TiC) or a complex compound involving elements titanium, boron, and carbon, as well as those elements contained in the substrate. In the case of cemented tungsten carbide, for instance, a complex compound can include the elements not only titanium, boron and carbon, but also the substrate elements of tungsten and cobalt.

The types of materials which can be surface treated by this invention include all classes of cemented carbide as follows:

(1) Tungsten carbide plus cobalt.

(2) Tungsten carbide plus cobalt plus titanium carbide.

(3) Tungsten carbide plus cobalt plus tantalum carbide.

(4) Tungsten carbide plus cobalt plus niobium carbide.

(5) Tungsten carbide plus cobalt plus titanium carbide plus tantalum carbide and/or niobium carbide.

(6) Titanium carbide plus nickel.

(7) Titanium carbide plus nickel plus molybdenum carbide.

3,744,979 Patented July 10, 1973 In the above list, other binder elements may be substituted for the cobalt or the nickel and suitable elements, which will work with the method outlined in this invention, include binder elements of cobalt, nickel or iron individually or in any combination. In addition the invention covers cemented carbides containing other carbide compounds than those outlined above, including vanadium carbide, chromium carbide, hafnium carbide and zirconium carbide and molybdenum carbide. The compounds which are used to form the surface treatment, which is the basic part of this invention, involves the combination of boron carbide B C) and titanium'hydride (TiH or TiH It is recognized that other compounds of a similar nature to titanium hydride may be used, such as zirconium hydride or hafnium hydride, and that the boron compound may be introduced in some other form which in the reaction becomes boron carbide. The salient features covered by this invention include the use of a boron containing compounds such as boron carbide, and the use of a reactive metal hydride, which will form a diffusion from the reactive ingredients into and onto the surface of the cemented carbide.

In addition to the use of a reactive metal hydride, such as titanium hydride, it is possible to use titanium metal powder as one of the ingredients and by virtue of having hydrogen in the furnace in which the reaction occurs, to form the titanium hydride in situ, which will result in the same diffusion and compound formation within and on the surface of the cemented carbide substrate.

The method of applying the compound involves the mixing of the boron carbide and titanium hydride powder intimately by ordinary mechanical means, placing a bed of the powder in a boat or other suitable container, placing the cemented carbide to be treated on the bed of powder and surrounding the piece with titanium hydride and boron carbide powder. The boat is then put through a furnace to cause the titanium hydride and boron carbide to react forming hard compounds on the surface of thecemented carbide substrate, which have special characteristics outlined in the examples to be shown below. The reaction is carried out in an atmosphere furnace containing a hydrogen atmosphere or an atmosphere of hydrogen and nitrogen such as dissociated ammonia. The reaction can also be carried out in an inert atmosphere or vacuum. The temperature of the reaction will be between 500 and 1500 C. and the time of reaction from 1 minute to 5 hours, depending upon the depth of diffusion desired for the surface treatment, as well as the particular cemented carbide grade being treated.

In all of these specific examples outlined a mixture of 50 percent by weight of 325 mesh titanium hydride powder and 50 percent by Weight of 325 mesh boron carbide powder was used. The mixture was formed by placing the two compound powders together in a closed cylindrical container placed at an angle to the horizontal axis and rotated for 1 hour. In all of the examples the cemented carbide pieces (inserts) were placed in a bed of said titanium hydride and boron carbide powder contained in a stainless steel boat. The boat was heated in a dissociated ammonia atmosphere for approximately /2 hour at a temperature of 850 C.

The following examples illustrate the effects of the surface treatment:

EXAMPLE 1 Five (5) throwaway machining inserts, consisting of a nominal composition of 77 weight percent tungsten carbide, 8 weight percent cobalt, 6.5 weight percent titanium carbide, and 8.5 weight percent tantalum carbide, were treated with the mixture of titanium hydride and boron carbide at 850 C. As a control 5 other inserts were placed in another boat where the powder consisted of the boron carbide powder alone, without the titanium hydride powder added.

The inserts were then submitted for machining test which were performed as follows: material, high tensile cast iron; type cut, facing; surface feet per minutes, 450; depth of cut, A speed per revolution, 0.025".

Under these machining conditions the untreated cemented carbide of the composition noted in this example, which was a standard grade and insert to use in this application, showed a performance of 60 to 100 pieces machined per corner. The pieces treated in the plain boron carbide as a controlled dummy run, were capable of machining only 40 pieces per corner. The pieces treated in the special mixture of boron carbide and titanium hydride showed a performance from a low of 251 pieces per corner on 1 insert, to a high of 365 pieces per corner on another insert, with an average of 297 pieces per corner for all inserts tested.

EXAMPLE 2 A series of inserts of a composition consisting of 74 weight percent tungsten carbide, 11 weight percent cobalt; 8 weight percent titanium carbide, and 7 weight percent tantalum carbide, were tested as follows: 5 inserts were treated with the 50 percent titanium hydride, 50 percent boron carbide mixture, 5 pieces were left in the untreated condition and a series of 5 pieces each were given four different types of the titanium carbide coating deposited by the titanium tetrachloride method.

Machining was done on pinions made of 4028 steel forgings. The operation was turning at 250 r.p.m. at 250 to 450 s.f.p.m. The depth of cut was to A and the feed per revolution was 0.014. The following results were obtained:

Average pieces per corner The inserts were checked to determine the difference in microhardness of the surface before and after the surface treatment. Using a 2 kg. load the hardness on the untreated cemented carbide was 1679 Vickers. After the above mentioned 50 percent boron carbide 50 percent titanium hydride treatment the surface hardness was increased to 1832 Vickers, Hardness Number.

EXAMPLE 3 Cemented carbide inserts in the following composition were tested in a machining application with and without the special surface treatment. The application was machining of a 12 ft. long integral piston rod in 4140 steel in the as hot rolled condition. The hardness of the steel was 30 Rockwell C. The operation was a turning operation using a Le Blond lathe of 25 HP. The operation was run at 250 s.f.p.m., with a feed of 0.018", and a depth of cut of .312". The diameter of the piston rod was 6".

The composition of the inserts used in this example was 75 weight percent tungsten carbide, 8 weight percent cobalt, 10 weight percent titanium carbide, and 7 weight percent tantalum carbide Using the untreated inserts of this composition it was possible on 1 corner of an insert, to turn 4 ft. of the piston rod which was not sufiicient to complete 1 piston rod. Utilizing the treated insert, it was possible to turn 42 ft. per corner of an insert and thus several piston rods could be completed utilizing only 1 corner of 1 insert.

EXAMPLE 4 Machining of laminated annealed steel for fractional horsepower electric motor rotors. The operation involved boring using cutter inserts. Three inserts are used in. the

machining operation with 1 insert boring 0.025" depth of cut, the second one 0.025 and the third one 0.008" depth of cut. A cemented titanium carbide composition was used for this application. The composition is more than two-thirds by weight percent of titanium carbide with the balance of the composition consisting of nickel and molybdenum carbide. The untreated insertswere capable of producing 6 to 14 rotors per edge prior to the need for indexing, whereas the titanium hydride-boron carbide treated inserts were suitable for producing 24 to 25 rotors per edge.

EXAMPLE 5 Machining of a type 4140 steel sand casting in the heat treated condition, with a hardness of 27 to 42 Rockwell C. This is a milling operation with a milling cutter using 6 cemented carbide inserts and operating at 279 r.p.m. This represents 290 s.f.p.m. The feed is from 3" to 8" per minute with a depth of cut of to g". The insert and the composition used consisted of 77 weight percent tungsten carbide, 8 weight percent cobalt, 6.5 weight percent titanium carbide, and 8.5 weight percent tantalum carbide. With I the usual untreated inserts it was only possible to machine 7 pieces per insert edge prior to the requirement for indexing. With the titanium hydride-boron carbide treated inserts it was possible to machine 22 pieces per edge prior to the need for indexing the insert.

All of the above relates to various machining applications on ferrous materials. The method will work equally well for machining many non-ferrous metals and because of the special hard surface will also enhance the wear re sistance of cemented carbides which are given this treatment.

The metallographic structure was studied to determine the effects of various compositions of titanium hydride and boron carbide in the mixture. The same structure was observed Where a composition of 80 percent by weight titanium hydride and 20 percent by weight boron carbide was used. Uniquely different structures were observed, although the surface was effected by the diffusion of new components into the surface, where 100 percent boron carbide or 100 percent titanium hydride was used in the unmixed condition. It is anticipated that the composition of the titanium hydride and boron carbide can be varied from approximately 5 percent by weight of titanium hydride, percent boron carbide, to 95 percent titanium hydride, 5 percent boron carbide. Utilizing differing percentages of each ingredient unmixed will also effect the surface condition of the cemented carbide and will effect the machining ability of the cemented carbide for some applications.

In the examples described above the inserts were placed in a bed of the mixed powder of titanium hydride and of boron carbide. Other methods of applying the material to the surface of the substrate to be surface treated may be used as well. This can include mixing the two integral powders in a vehicle to form a slurry and painting the surface of the cemented carbide, or dipping the piece of cemented carbide into slurry to coat the surface with the material being treated.

Examples described above involve the use of this treatment for cutting tools but it is well known that the in-- crease in hardness or other improvement of the surface of a metal, will make it suitable for many other parts where a high hardness surface and improved wear or abrasion resistance is required. Examples of this type of application would be in wire drawing dies, powder compacting molds and punches, and other applications where cemented carbide or hardened steel is used to decrease the wear of parts of machinery. I also claim the use of this surface treatment for improving the corrosion or oxidation resistance of metals upon which it may be applied. In addition to the use of this surface treatment for cemented carbides, I claim its applicability for other metals which form borides or carbides, such as the ferrous metals, metallic titanium and its alloys, metallic zirconium and its alloys, as well as other reactive and refractory metals.

Boron carbide forming material may comprise boron powder mixed with and carbon powder; boron oxide mixed with carbon powder; or boric anhydride mixed with carbon.

Titanium hydride forming material may comprise titanium powder in presence of a hydrogen containing atmos phere.

I claim:

1. A method of treating the surface of a piece of cemented carbide, comprising treating said piece for at least one minute at a temperature above 500 C., with one of the class selected from boron carbide and boron carbide forming material comprising boron and carbon; together with the hydride of a refractory metal selected from titanium, zirconium and hafnium.

2. The process of claim 1, wherein the treatment of said piece is carried out in the presence of one of the class comprising hydrogen, dissociated ammonia, inert gas, hydrocarbon gas and vacuum.

3. The process of claim 1 wherein the cemented carbide further comprises one of the class ferrous, refractory metals and metals which form carbides and/or borides.

4. The process of claim 1, wherein the piece of cemented carbide comprises one of the class selected from tungsten carbide plus cobalt; tungsten carbide plus cobalt plus titanium carbide; tungsten carbide plus cobalt plus tantanlum carbide; tungsten carbide plus cobalt plus niobium carbide; tungsten carbide plus cobalt plus titanium carbide plus tantalum carbide and/or niobium carbide; titanium carbide plus nickel; and titanium carbide plus nickel plus molybdenum carbide.

5. The process of claim 1, wherein the piece of cemented carbide comprises one of the class comprising tungsten carbide and titanium carbide, together with a metal binder selected from cobalt, nickel and iron.

6. The process of claim 1, wherein the treatment is in a furnace containing an atmosphere of hydrogen.

7. The process of claim 1 wherein the boron carbide and the hydride are in the form of powder, and wherein the piece treated is in a bed of said powder during heating.

8. The process of claim 1, wherein the piece treated is covered with the materials with which it is being treated, and wherein said materials are in the form of a slurry applied to the surface of said piece prior to heating.

9. The process of claim 1, wherein the heating is between 500 C. and 1500 C.

10. The process of claim 9, wherein the heating lasts about one half hour.

11. The process of claim 1 wherein the mixture is 5% to 95% by weight of boron carbide and 95 to 5% by weight of the hydride.

12. The product produced by the process of claim 1.

13. The combination of claim 12, said mixture being 5% to 95% by weight of said boron carbide or boron carbide forming material and 95 to 5% of said hydride.

14. A method of treating the surface of a piece of cemented carbide, comprising treating said piece for at least one minute at a temperature above 500 C., with one of the class comprising boron carbide and boron carbide forming material comprising boron and carbon; plus one of the class comprising the hydride of a refractory metal selected from titanium, zirconium and hafnium and material which upon heating above 500 C., will form a hydride of said refractory metal in situ, in the presence of an atmosphere comprising hydrogen.

References Cited UNITED STATES PATENTS 3,647,576 3/1972 Yamamura et a1. 203 X 3,178,273 4/1965 Libal 51307 X 3,003,860 10/1961 Sermon et al 51307 2,351,798 6/ 1944 Alexander 117--22 2,711,980 6/ 1955 De Santis et al 75204 X 2,512,455 6/1950 Alexander 11722 X 3,001,893 9/1961 Kreuchen et al 117-22 X ALFRED L. LEAVI'IT, Primary Examiner J. R. BA'I'IEN, 1a., Assistant Examiner U.S. Cl. X.R.

29-1827, 182.8; 51-307; 1l7-22, 127, 169 R, Dig. 10; 1486, 6.14 R; 106-286 

